1. Field of the Invention
The present invention relates to a pressure sensor, a pressure control apparatus and a pressure-type flow rate control apparatus for use mainly in semiconductor manufacturing facilities, chemical plants, etc. More particularly, the present invention concerns apparatus for correcting temperature drift—for use in a pressure sensor, a pressure control apparatus, and a pressure-type flow rate control apparatus—which accurately detects the pressure of a fluid by automatically negating or correcting a temperature drift, thus controlling the pressure and flow rate of fluid, where the output of a pressure sensor for measuring the pressure of the fluid drifts as a result of a temperature change.
2. Description of the Prior Art
In semiconductor manufacturing facilities, chemical plants, etc., a plurality of gases as materials are commonly supplied at specific flow rates, and are reacted in a reaction vessel to produce an object or target gas. In this process, if the material gases are not supplied accurately, then the chemical reaction proceeds unevenly, and it can occur that material gases remain unreacted in the produced object gas. Especially, where the material gases are flammable, there is a danger of gas explosions. In order for the material gases to react in a well-balanced manner, it is necessary to control accurately the flow rates of the gases to be supplied.
Hitherto, the flow rates of gases have been controlled through the use of the following arrangements: an orifice is mounted in a pipe, a theoretical flow rate formula is selected that can indicate as accurately as possible the flow rate of a gas passing through said orifice, and the flow rate of the gas passing through the orifice is calculated using the flow rate formula. In the flow rate formula that has been used hitherto, the fluid is assumed to be incompressible, and the flow rate Qc is expressed as:Qc=KP21/2(P1−P2)1/2
where P1 is the pressure on an upstream side of the orifice,
P2 is the pressure on an downstream side of the orifice, and
K is a proportional constant which depends on the fluid temperature.
In the flow rate formula, the flow rate is calculated from two pressure parameters P1 and P2. However, since the actual gas flow comprises a compressible fluid, the above theoretical flow rate formula is not very precise. On the other hand, if the ratio P2/P1 of above-mentioned pressures is reduced below a critical value of about 0.5, then the flow velocity of the gas passing through the orifice reaches sonic velocity, and it is known that under sonic velocity conditions, the theoretical formula is:Qc=KP1
It is known that as long as critical conditions are satisfied, the flow rate simply depends only on the upstream side pressure P1, and yields accurately the rate of a compressible fluid passing through the orifice.
For approximate flow rate control of an incompressible fluid, therefore, the theoretical flow rate formula:Qc=KP21/2(P1−P2)1/2
is used. Under critical conditions (P2/P1<about 0.5), the theoretical flow rate formula:Qc=KP1
is predominantly used for controlling the flow-rate of a fluid. Where either of these flow rate formulae is used, the measurement of the fluid pressures, P1 and/or P2, is a prerequisite. That is, in the case of former, the simultaneous measurement of P1 and P2 is necessary, and in the latter case the measurement of the upstream side pressure P1 is required.
In order to measure fluid pressure, it is necessary to install a pressure sensor in the fluid. The pressure sensor therefore becomes very sensitive to the fluid temperature, and the sensor temperature immediately becomes equal to the fluid temperature T. In other words, equilibrium between the fluid temperature T and the temperature of the sensor is immediately established. In order to measure the fluid pressure accurately, it is necessary to reduce the size of the pressure sensor to such an extent that the flow of the fluid is undisturbed. Accordingly, it will be appreciated that equilibrium is reached very quickly.
On the other hand, the gaseous fluid flowing through the pipe is controlled so that the fluid flows as far as possible at a constant temperature. However, it is known that over a period of many hours, the temperature of a flow of gas fluctuates considerably. Furthermore, when a gas fluid is replaced by another gas, it can happen that an high temperature gas flows for a certain period of time, whilst a low temperature gas flows for another period of time. Accordingly, if a fluid of fluctuating temperature is measured using the same pressure sensor, then the temperature drift characteristics of the pressure sensor output present a problem such that the detected pressure of the fluid needs to be corrected.
Prior art pressure sensors invariably display a temperature drift, irrespective of the method of pressure detection. “Temperature drift” means that when the ambient temperature around the pressure sensor changes, the output of the pressure sensor changes under constant pressure. It is found that this output drift correlates to the fluid temperature.
Some pressure sensors have a built-in temperature compensation circuit, but even here, if the temperature drift is 0.05%/° C., for example, then an output drift of 4% will occur when the temperature changes from 20 to 100° C.
Pressure sensors are available in various different types. Consider a strain gauge for example: A strain gauge converts pressure into a voltage such that with pressure plotted on an abscissa axis, the ordinate axis corresponds to output voltage. Of course, if the absolute pressure is zero, then the output voltage will be zero, and it is expected that with an increase in absolute pressure, the output voltage will rise.
The sensor output recorded when zero pressure is applied to the pressure sensor is called the “zero-point output”. The temperature drift of the zero-point output, which is the fluctuation in the zero-point output occurring in accordance with temperature change, is called the “zero-point pressure drift”. On the other hand, the temperature drift of the sensor output observed when pressure is applied is called the “span output drift”. In order to obtain an accurate sensor output, it is necessary to adjust for both zero-point output drift and span output drift.
More specifically, assume that the pressure sensor has no zero-point output drift, and that its zero-point voltage is 0 (V). Also assume that when the absolute pressure of the fluid is 1.0 (×102 kPaA), i.e. 1 atm, the output voltage is 20 mV. When the fluid temperature changes under these conditions, the output voltage naturally deviates from 20 mV. This fluctuation is the above-mentioned span output drift. In practice, since some zero-point output is always present, the span output drift at any arbitrary pressure is displaced by the zero-point voltage.
As described above, in a pressure-type flow rate control apparatus that controls a fluid passing through an orifice by measuring an upstream side pressure P1 or a downstream side pressure P2, temperature fluctuation characteristics called the zero-point output drift and the span output drift are included in the output voltage of the pressure sensor. If the output voltage is converted directly into a voltage, therefore, the pressures P1, P2 will contain errors. If the flow rate is calculated in accordance with the aforesaid flow rate formula, then those errors will be introduced into the calculated flow rate, Qc. This is the problem of temperature drift in pressure-type flow rate control apparatus.
The zero point and span temperature drift characteristics of a pressure sensor are different as between different pressure sensors. It is desirable to obtain an accurate sensor output by providing a method of correcting the temperature drift.